Measurement of radioactivity of soil -Reply

[FRAMEP@ORAU.GOV]

>I have an MSc student who wants to carry out a small project to 
>relate radon levels in soil (previously measured by another student)
>to the uranium and thorium content of soil samples from a number of
>sites in the UK.  In particular we want to investigate the dependence 
>of radioactivity in soil to soil particle size.  Our initial thoughts were to
>use GeLi gamma spectrometry.  I would value some references to
>previous studies and in particular some guidance on preparation of
>samples, calibration methods and required counting times.

Make sure the soils are well dried and mixed.  Twelve hours at 110
degrees C should do it for the drying.  500 ml marinelli beakers should be
adequate but be sure that the beakers are always filled to the same
level. If you are also going to look at the radon daughters, its probably
best to completely fill the container so that there is no open space near
the top where the radon could accumulate. Also, when looking at the
radon daughters, the container has to be sealed and equilibrium has to
be established. Count times for uranium could be long i.e. 4 or more
hours. The catch is that the uranium is quantified via the gammas from
Th-243 and Pa-234m and these have very low yeilds. Thorium is not so
bad because you are looking at the gammas from Ac-228. I'd quantify the
uranium separately using the 63 line from Th-234 and the 1001 line from
Pa-234. The beauty of the 1001 line is that it will show little self
absorption. Unfortunately it has an even lower yeild than Th-234's 63
keV gamma. The trick is that use of the 63 keV line requires self
absorption corrections. To do this, some folk make a jig containing
Am-241(59.5 keV gamma) sources that is placed over the marinelli. Two
counts are made with and without the jig in place. A fairly simple
equation is then used to get a correction factor. Self absorption
corrections wouldn't be required if all soils were the same but they can
differ in density as well as elemental make-up. Some labs use two
efficiency curves for soil. One for high density soil, and the other for low
density soil. The density is simply determined from the weight required to
fill the soil container. To do this, of course, you need multiple soil
standards, something thats not necessary with the use of Am-241(for
example). Furthermore, this approach only partially accounts for density
differences and doesn't really address differences in elemental
composition of the soil. BTW I'm pretty sure that you can get some nice
inexpensive soil standards from the IAEA. 

I hope that helps a little. Unfortunately, this isn't a nice straightforward
type of problem. Interpreting the results might not be a piece of cake
either.

Paul Frame
Professional Training Programs
ORISE
framep@orau.gov